1. Field of the Invention
The present invention relates to a method of transmitting data to multi destinations in a wireless local area network (WLAN) system, and more particularly, to a method of transmitting data to multi destinations in a WLAN system, which can minimize an access time to a media in a contention-based media access control network by transmitting an aggregated-media access control protocol data unit (A-MPDU) having a plurality of MPDUs addressed to different destinations and sequentially receiving response frames (ACK frames) according to a given order.
2. Description of the Related Art
Recently, IEEE 802.11n standard is close to being finalized, which is expected to provide a high data transmission rate beyond 100 Mbps in a user area. Accordingly, a home network system, which can transmit a high definition (HD) moving picture (HD stream) with high capacity using a high bandwidth, is being actively researched and commercialized.
FIG. 1 illustrates a general home network using a wireless AV system. Referring to FIG. 1, an access point AP of a wireless local area network (WLAN) system includes an AV stream source generator 11 generating AV stream data, a wireless transceiver 12 transmitting the generated AV stream data wirelessly. Stations STA1, STA2 and STA3 of the WLAN system respectively include wireless transceivers 13-1, 13-2 and 13-3 receiving data transmitted from the wireless transceiver 12 of the access point AP, and displays 14-1, 14-2 and 14-3 displaying the received data.
Typically, in case of data composed of image data such as HD moving picture, a user can promptly recognize a display state when data is partially lost. Also, when an image data reaches at a time later than a given time, an image is abnormally displayed. Therefore, to satisfy service quality for a user, it is very important to minimize a delay time of data transmitted from the access point AP to each of the stations STA1 to STA3. In particular, there is an increasing demand for a method of transmitting data, which can reduce a delay of each station in a WLAN system where one access point AP is connected to multi stations STA1 to STA3.
In general, IEEE 802.11 WLAN technology employs a distributed coordination function (DCF) based on carrier sense multiple access with collision avoidance (CSMA/CA) so as to allow multi users to access to a wireless channel. According to a backoff scheme of the DCF, a wireless device transmits data if a channel is not used, after detecting for a random time whether the channel is used prior to the transmission of the data. This allows several wireless devices to share the same wireless channel. Further, the IEEE 802.11 WLAN technology uses a response frame of an ACK frame so as to provide the accuracy of data transmission due to characteristics of a wireless channel. When data addressed to a specific destination is transmitted, and a wireless device having a corresponding destination address receives the data accurately, an ACK frame is transmitted to the wireless device transmitting the data. If, however, the wireless device transmitting the data does not receive the ACK frame in response to the transmitted data, the wireless device judges that the transmitted data is lost or damaged due to channel conditions, and re-transmits the data. Hence, the WLAN system provides the accuracy of data transmission through the above-described procedure.
FIG. 2 illustrates a related art method of transmitting data from the access point AP to two stations STA1 and STA2 according to the IEEE 802.11 WLAN technology. According to the IEEE 802.11 WLAN technology, a media access control (MAC) layer of the access point AP of the WLAN system generates data to be transmitted to each station as a media access control protocol data unit (MPDU). Further, the MAC layer of the access point AP adds a physical header (PHY) to an MPDU to be transmitted to each station. Thereafter, the access point AP transmits the MPDU, which is addressed to the first station STA1 and the physical header (PHY) is added to, over a wireless channel, after the total lapse of a using time of the wireless channel (Busy Medium), a DCF interframe space (DIFS) and a backoff time (Backoff). The access point AP receives the ACK frame from the first station STA1 after the lapse of a predetermined short interframe space (SIFS) from a timing when the transmission of the MPDU to the first station STA1 is completed. Subsequently, the access point AP transmits an MPDU, which is addressed to the second station STA2 and a physical header (PHY) is added to, over the wireless channel, after the total lapse of a DIFS and a backoff time. After the lapse of an SIFS, the access point AP receives an ACK frame.
According to the related art method of transmitting data to multi destinations, as described above, the DIFS, backoff, SIFS, and ACK frame reception time should be required whenever the MPDUs are respectively transmitted to the stations. This causes the transmission of an MPDU to a next destination station to be delayed. Particularly, if the access point does not receive the ACK frame when an MPDU is transmitted to a preceding destination station, the access point should re-transmit the MPDU to that preceding destination station. Consequently, the transmission of an MPDU to a next destination station is further delayed.
To solve the transmission delay, a method of broadcasting data to be transmitted from the access point to the station may be used because it can simultaneously transmit data to all the stations through one-time data transmission so that it is possible to minimize a delay time caused by a transmission order of MPDUs for each station. However, this method is disadvantageous in that it is impossible to increase the accuracy of data transmission through re-transmission because there is no method of detecting the loss of broadcasted data occurring during transmission. Moreover, the broadcasting method is available only when all the stations request the same data.
Therefore, it is essentially required to develop a new transmission method, which can increase the accuracy of data transmission through re-transmission while transmitting data to multi destinations in a WLAN system.